The present invention relates generally to switching power supplies, and more particularly to a switching power supply having a continuous-mode master buck converter and a discontinuous-mode buck-boost slave converter.
Switched-mode power supplies (xe2x80x9cconvertersxe2x80x9d) alternately store and output energy. Converters receive either a DC or rectified AC voltage as an input. Energy from the input voltage is temporarily stored in an inductor during each switching cycle. An oscillating switch in converters is actuated to control how much of the energy is output. A filter is used to smooth the output into a DC voltage and current. The output DC voltage can be higher or lower than the input voltages. The output DC voltage may also be negative with respect to the input voltage.
Converters operate in either a discontinuous mode or a continuous mode. In the discontinuous mode, converters completely de-energize the inductor before the end of every switching cycle. Thus, there is no current in the inductor at the start of every switching cycle in the discontinuous mode. In the continuous mode, converters do not completely de-energize the inductor before the end of every switching cycle. Thus, the current in the inductor never reaches a point where there is no current in the inductor in the continuous mode.
The output of a converter is determined in part by the duty ratio. The duty ratio is equal to the time period in which the switch is xe2x80x9conxe2x80x9d divided by the time period of the switching cycle (D=Ton/T). The switching cycle time period is equal to the time period in which the switch is xe2x80x9conxe2x80x9d plus the time period in which the switch is xe2x80x9coffxe2x80x9d (T=Ton+Toff).
There are three basic topologies of converters: the buck converter, the boost converter, and the buck-boost converter. The output voltage of the buck converter is equal to the input voltage multiplied by the duty ratio (Vout=Vin*D). Thus, the output voltage of the buck converter cannot be greater than the input voltage. Also, the output voltage of the buck converter cannot be negative.
In the boost converter (or xe2x80x9cstep-upxe2x80x9d converter), the output voltage is equal to the input voltage times the switching cycle time period divided by the time period in which the switch is off (Vout=Vin*T/Toff). Thus, the output voltage of the boost converter cannot be less than the input voltage. Also, the output voltage of the boost converter cannot be negative with respect to the input voltage.
The buck-boost converter has an output voltage that has a polarity that is negative with respect to the input voltage. The output voltage is equal to the negative value of the input voltage multiplied by the ratio of the time period in which the switch is on to the time period in which the switch is off (Vout=xe2x88x92Vin*Ton/Toff). Thus, the output voltage has a magnitude that is either stepped-up or stepped-down from the input voltage as determined by the duty ratio.
Transformer-coupled converters can be derived from the basic converter topologies. The inductor of a basic converter is replaced with a transformer in a transformer-coupled converter topology. Transformer coupling provides the advantages of DC isolation. The polarity of the output can be changed by reversing the connections to the secondary winding of the transformer. Additionally, the output voltage can be scaled by adjusting the ratio of the primary to secondary windings of a transformer.
The flyback converter is a buck-boost-type converter that has a transformer that replaces the simple inductor of the buck-boost converter. Thus, the transfer function (i.e., the relationship of the output voltage to the input voltage) of the flyback converter is similar to the buck-boost converter. The output voltage of flyback converter has a magnitude that is either stepped-up or stepped-down from the input voltage.
The forward converter is a buck-type converter that has a transformer that replaces the simple inductor of the buck converter. Thus, the transfer function of the forward converter is similar to the buck converter. The output voltage of the forward converter is equal to the input voltage multiplied by the duty ratio.
The present invention is directed to an efficient, well-regulated slave buck-boost converter that is driven from a master buck converter. The derived transfer function of the slave buck-boost converter that is being driven from a master buck converter is not dependent upon the input voltage. Thus, line regulation is automatically achieved. Load regulation is achieved by using a shunt regulator at the output of the slave buck-boost converter. The shunt regulator is arranged such that operating the slave buck-boost converter at its designed maximum load is almost as efficient as a conventional buck-boost converter. The slave converter is operated in discontinuous mode so that changing the inductance of the slave converter provides a desired voltage output.
According to one aspect of the invention, the power supply for receiving an input voltage and providing a main output voltage and an auxiliary output voltage comprises a continuous mode master converter, and a discontinuous mode slave converter. The continuous mode master converter is configured to provide the main output voltage, and includes a controller that is configured to produce a modulated signal at a controller output node for regulating the main output voltage. The main output voltage is determined by a product of the input voltage and a duty ratio of the modulated signal. The discontinuous mode slave converter is configured to provide the auxiliary output voltage, and is coupled to the controller output node. The auxiliary output voltage is determined by a product of the input voltage and a ratio of the time period in which the modulated signal is in an xe2x80x9conxe2x80x9d state to the time period in which the modulated signal is in an xe2x80x9coffxe2x80x9d state.
According to another aspect of the invention, a method for providing a main output voltage and an auxiliary output voltage comprises using a controller in a continuous mode master converter to provide a modulated signal. The continuous mode master converter is used to provide the main output voltage in response to the modulated signal. The modulated signal is coupled to a control terminal of a discontinuous mode slave converter. The continuous mode slave converter is used to provide the auxiliary output voltage in response to the modulated signal.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrated embodiments of the invention, and to the appended claims.